Method for catalytically producing formic acid and regenerating the catalyst used in the process with little overpressure

Abstract

The invention relates to catalytically producing formic acid and regenerating the catalyst used in the process. A vanadyl ion, vandate ion, or polyoxometallate ion, which is used as the catalyst, of the general formula [PMoxVyO40]n− is brought into contact with an alpha hydroxyl aldehyde, an alpha hydroxy carboxylic acid, a carbohydrate, a glycoside, or a polymer, which contains a carbon chain and which comprises at least two OH groups bonded as substituents to the carbon chain as a substituent in a repeating manner and/or an O, N, or S atom contained in the carbon chain in a repeating manner, in a liquid solution (12) in a vessel (10) at a temperature above 70° C. and below 160° C., wherein 6≤x≤11, 1≤y≤6, 3<n<10, and x+y=12, where n, x, and y is each a whole number.

The invention claimed is: 
     
       1. A method for catalytically producing formic acid and regenerating the catalyst employed, where a vanadyl ion, vanadate ion or polyoxometalate ion of the general formula [PMo x V y O 40 ] n−  serving as catalyst is contacted at a temperature above 70° C. and below 160° C. with an alpha-hydroxyaldehyde, an alpha-hydroxycarboxylic acid, a carbohydrate, a glycoside or a polymer containing a carbon chain and having at least two OH groups bonded as substituents to the carbon chain and/or having an O, N or S atom present repeatedly in the carbon chain, in a liquid solution in a vessel, where 6≤x≤11 and 1≤y≤6 and 3<n<10 and x+y=12, where n, x and y are each an integer, where the catalyst reduced is returned by oxidation to its original state, characterized in that the liquid solution for this purpose is contacted with a gas comprising a volume fraction of at least 18% of oxygen at a pressure in a range of 2 to 16 bar, by means of a mixing apparatus or via a liquid-impermeable, gas-permeable membrane, where CO and/or CO 2  formed in the reaction and passing into the gas are/is taken off in a quantity such that the volume fraction of CO and CO 2  together in the gas does not exceed 80%. 
     
     
       2. The method as claimed in  claim 1 , characterized in that the volume fraction of CO and CO 2  together in the gas is at least 20% and/or in that the CO and/or CO 2  formed in the reaction and passing into the gas are/is taken off in a quantity such that the volume fraction of CO and CO 2  together in the gas does not exceed 70%. 
     
     
       3. The method as claimed in  claim 1 , characterized in that the CO and/or CO 2  formed in the reaction and passing into the gas are/is taken off in a quantity such that the volume fraction of CO and CO 2  together in the gas does not exceed 80% by using fresh gas to replace the gas contacting the solution, or at least a part of this gas, permanently or intermittently, no later than on attainment of the volume fraction of 80%, or by separating the CO and/or the CO 2  from the gas. 
     
     
       4. The method as claimed in  claim 1 , characterized in that the vanadyl ion, vanadate ion or polyoxometalate ion is contacted at a temperature above 80° C., with the alpha-hydroxyaldehyde, the alpha-hydroxycarboxylic acid, the carbohydrate, the glycoside or the polymer. 
     
     
       5. The method as claimed in  claim 1 , characterized in that the pressure is at least 3 bar. 
     
     
       6. The method as claimed in  claim 1 , characterized in that the CO and/or CO 2  formed in the reaction and passing into the gas are/is taken off in a quantity such that, with a pressure restricted to a maximum value or with pressure held constant, the oxygen partial pressure in the gas is diminished by the CO and/or CO 2  by not more than 10%. 
     
     
       7. The method as claimed in  claim 1 , characterized in that for the oxidation of the catalyst, a portion of the liquid solution is led out of the vessel, contacted with the gas, and subsequently supplied again to the remainder of the liquid solution. 
     
     
       8. The method as claimed in  claim 1 , characterized in that vaporous formic acid formed in the gas is absorbed from the gas by means of an absorbent suitable for absorbing formic acid, or an amide and is subsequently desorbed therefrom, or is absorbed from the gas by a base and the resulting salt solution is led off or is condensed at the vessel or outside the vessel, where a condensate formed as a result is led back into the vessel, supplied to an extraction to separate the formic acid from the water contained therein, or led off for the separation of the formic acid. 
     
     
       9. The method as claimed in  claim 1 , characterized in that the mixing apparatus comprises a static mixer, a reactive mixing pump, a nozzle and/or a gas introduction stirrer. 
     
     
       10. The method as claimed in  claim 1 , where the mixing apparatus is designed in such a way and operated in such a way, or the membrane is constructed in such a way, that the surface area of the solution is increased by a factor of at least 1000 as a result. 
     
     
       11. The method as claimed in  claim 1 , characterized in that the CO and/or the CO 2  are/is separated from the gas by means of a membrane which is permeable for the CO and/or the CO 2  and impermeable or of only limited permeability for O 2 , a combination of two or more membranes with different permeabilities, or by means of a pressure swing adsorption. 
     
     
       12. The method as claimed in  claim 1 , characterized in that the gas is guided in a circuit. 
     
     
       13. The method as claimed in  claim 1 , characterized in that the method is carried out as a continuous process. 
     
     
       14. The method as claimed in  claim 1 , characterized in that the catalyst and the formic acid are separated from the liquid solution or from a portion of the liquid solution that is subsequently resupplied to the liquid solution, by means of at least one polar organic extractant which extracts the formic acid and the catalyst and which, on mixing with the liquid solution, forms a phase boundary between the liquid solution and the extractant, in the liquid solution, where the extractant is one which, for extraction of the catalyst present at a concentration of 1.5 wt % in water, has a partition coefficient for the catalyst at 40° C. that is greater by a factor of at least 7 than a partition coefficient for extraction of the formic acid present at a concentration of 5 wt % in water at 40° C., and where the extractant before the extraction is saturated with the catalyst or where the separated catalyst is separated from the extractant and resupplied to the liquid solution in the vessel. 
     
     
       15. The method as claimed in  claim 14 , characterized in that the separation takes place in a two-stage process, by extracting the liquid solution in a first extraction step with a first quantity of the extractant for a first time, to extract the catalyst, and extracting the solution in a second extraction step with a second quantity of the extractant for a second time, to extract the formic acid, where the catalyst extracted in the first extraction step is supplied again to the liquid solution in the vessel. 
     
     
       16. The method as claimed in  claim 14 , characterized in that the extractant is admixed with an additive. 
     
     
       17. The method as claimed in  claim 14 , characterized in that the extractant is an amide. 
     
     
       18. The method as claimed in  claim 14 , characterized in that the catalyst is separated by means of precipitation as a salt or by means of further extraction with a polar further extractant and with a temperature change of the extractant and/or an increase in the pH of the extractant. 
     
     
       19. The method of  claim 4 , wherein the temperature below 150° C. 
     
     
       20. The method of  claim 5 , wherein the pressure is at most 15 bar. 
     
     
       21. The method of  claim 8 , wherein the absorbent is a linear alcohol or an amide. 
     
     
       22. The method of  claim 9 , wherein the nozzle is a Venturi nozzle or a spraying nozzle. 
     
     
       23. The method of  claim 12 , wherein the gas is guided in the circuit without a pressure drop of more than 2.5 bar.